There is a need in the art for ammonia sensors that can detect and measure NH3 at temperatures higher than 500° C. for emissions control systems. Typically, for control applications, the accuracy of the measurement needs to be ±1 ppm, and the detection limit needs to be as low as 1 ppm. A review of pertinent patent and other literature revealed that currently known and used ammonia sensors are incapable of proper function at temperatures higher than 500° C. while providing a detection limit of 1 ppm. Techniques proposed for improving gas selectivity and sensitivity include the use of a polymer molecular sieve. These techniques inherently preclude use at high temperatures, since polymers are not stable chemically at such temperatures.
Optical sensors for the detection of NH3 include IR detectors and optic-fiber-based sensors. Optical sensors can generally provide accurate gas measurement with little cross-sensitivity to other gas constituents. For optical systems, however, the gas inputs must be transferred to an analysis chamber, resulting in long lag times. Further, the associated equipment for such optical sensors is generally bulky and highly expensive. In addition, the use of polymer/volatile sensing materials necessitates relatively cool gas temperatures (i.e., generally <100° C.).
Semiconductor sensors are one variety of currently-used sensors that are typically based on semiconductors such as metal oxides or polymers, and measure the change in resistance or capacitance of the coating as a function of adsorbed species. The primary problem with semi-conductor oxides in general is that they measure bulk properties based on adsorption of gases, and there is a significant issue of cross-contamination as all gases tend to adsorb on high-surface area ceramic substrates to some extent, resulting in significant errors in measurement. The main problem for ammonia measurements in engine exhaust streams is cross-contamination with carbon monoxide (CO), and oxides of nitrogen (NOx). To overcome this problem, one approach that has been tried is to use an “electronic nose” based on a number of semiconductor sensors operating in parallel that generate a series of responses in the presence of a mixture of gases. This results in the requirement for a very complex electronics package to calculate out the NH3 concentration, which is undesirable and cost ineffective.
Another problem faced in semiconductor sensors is that they have a low maximum temperature for use. Polymer-based sensors are useful only at temperatures below which the polymers are chemically stable (generally lower than 150° C.). Metal oxide semi-conductor sensors are typically most sensitive around 300° C., and they generally lose their sensitivity above 450° C., since the adsorption of most gases tails off above that temperature. Further, it has been observed that in many circumstances, semiconductor sensors typically have a long response time to fluctuations in ammonia concentration since they are kinetically limited by gas adsorption. The sensor responses of the series of sensors can then be analyzed to extract out information about the various gas species.
This approach has two challenges: (1) the limited temperature capability of semiconductor based sensors (generally less than 450° C.) and (2) the complexity of accompanying electronics required to extract out meaningful gas concentrations from the signals of various sensing elements. Generally, these types of sensors are more suitable for air quality monitoring rather than for engine control.
An attractive alternative is for exhaust gas hydrocarbon monitoring are solid-state electrochemical ceramic sensors. These devices can be broadly categorized into potentiometric and amperometric sensors, based on whether the monitored parameter is electrochemical potential or the current through the device at a fixed applied potential, respectively. Potentiometric sensors can be further categorized into equilibrium-potential-based devices and mixed-potential-based devices. There are three main categories of equilibrium-potential-based sensors, originally categorized by Weppner as Type I, Type II, and Type III sensors. The classification is relative to the nature of the electrochemical potential, based on the interaction of the target gas with the device. Type I sensors generate a potential due to the interaction of the target gas with mobile ions in a solid electrolyte (e.g. O2 sensors with yttria-stabilized zirconia-YSZ, an O2− ion conductor), whereas Type II sensors generate a potential due to the interaction of a target gas with immobile ions in a solid electrolyte (e.g. sensors based on CO2—K+ ion interaction). Type III sensors show no such direct relationship without the assistance of an auxiliary phase. Type II and Type III sensors are clearly unsuitable for high-temperature applications due to the nature of the materials used, generally nitrates, which are unstable and sometimes explosive at high temperatures. Type I sensors for NH3 sensing are feasible, but impractical. Due to the presence of oxygen in the exhaust stream, which would interfere with the measurement, elaborate pumping cells are required for removing the oxygen prior to gas sensing. This makes the device complex and increases operating costs to the point where it is not an attractive option. The same problem of initial oxygen removal exists for amperometric devices for gas sensing.
Amongst electrochemical sensors, the best option for exhaust gas monitoring to date has been mixed-potential based ceramic sensors. While this patent is directed at ammonia sensing and the key elements are the use of the catalyst system, the eventual species detected is NOx and the discussion of mixed potential sensors for NOx detection is relevant. Early work was performed by a Japanese group headed by Yamazoe and Miura on mixed potential sensors primarily for detection of NOx. Mixed potential sensors, which consist of metal, metal oxide or perovskite sensing electrodes on an oxygen ion conducting membrane, have a number of properties that make them very attractive for use as exhaust gas NOx sensors. They can operate effectively at temperatures as high as 650° C. Further, they do not require elaborate pumping cells for removal of oxygen and can be fabricated in very compact shapes using relatively easy and cost-effective conventional ceramic processing techniques such as isostatic pressing, sintering, ink-processing, electrode application and post-firing.
Thus, it would be an improvement in the art to provide methods and alternative configurations for ammonia-sensing systems designed to address these and other considerations. Such methods and devices are provided herein.